1. Field of the Invention
The present invention relates to a printed-wiring board and an electronic device having the same wiring board, of which a connection structure of the printed-wiring board in the electronic device is contrived to restrain radiant noises out of the electronic device provided with the printed-wiring board.
2. Related Background Art
It has hitherto been conceived preferable that a ground pattern provided on a printed-wiring board be so designed as to reduce fluctuations in electric potential. This intends to restrain an unnecessary current from being generated subsequent to the fluctuations in the ground potential on the wiring board. A first conventional method for this purpose is that the ground on the printed-wiring board might be connected to a box-like conductive member with a less amount of fluctuations in the potential by a screw or soldering or a conductive tape with a low impedance.
When the ground on the printed-wiring board is connected to the box-like conductive member simply through the low impedance, however, standing-waves occur at a given frequency due to the electric current flowing across the wiring board. The standing-waves are defined as intensive electromagnetic waves and might cause a problem of radiant noises in some cases. The above frequency is determined mainly by a higher harmonic frequency of a digital clock, a size of the wiring board, and a dielectric constant of a dielectric member constituting the wiring board.
Further, when connected to the box-like conductive member, there might be a case where the current is easy to flow across a connecting portion therebetween because of this connecting portion having a low impedance. The radiant noises are caused by the current flowing to the box-like conductive member as the case may be.
A second method of restraining the radiant noises caused by the standing-waves is disclosed in Japanese Patent Application Laid-Open No. 7-225634, wherein the connection is made at a plurality of portions which are given a resistivity.
If the box body (housing) shaping a configuration of the device is structured to incorporate two printed-wiring boards 52a, 52b as shown in FIG. 20, however, it might happen that a conductive member 51 of the box body exists in the vicinity of one printed-wiring board 52b, while the box body conductive member 51 facing to the wiring board 52a does not exist in the vicinity of the other printed-wiring board 52a. Further, even when there exists the conductive member 51 facing thereto, this conductive member does not have a sufficient size as compared with the printed-wiring board.
In such a case, even when trying to making the connection at the plurality of portions by use of the second conventional method, it might be impossible to provide the connecting portions in some cases. On the other hand, even if the ground on the wiring board is connected to the conductive member of the box body through an impedance, the standing waves can be restrained. However, this does not lead to stabilization of the ground potential on the wiring board. Namely, if the conductive member of the box body is not in the position facing to the wiring board in the vicinity of the printed-wiring board, it is unfeasible to restrain the radiant noises due to the fluctuations in the ground potential on the printed-wiring board.
Moreover, the ground pattern provided on the printed-wiring board is classified according to applications into those for, e.g., an analog signal and a connection to the box body, and the printed-wiring board is thus designed. Further, a power source pattern provided on the printed-wiring board is also divided corresponding to a drive voltage of a circuit in many cases.
If the ground pattern and the power source pattern exist in mixture as described above, the wiring and component layout designed on the printed-wiring board become intricate, and the ground on the wiring board and the conductive member of the box body are connected to each other in an optimal position with a difficulty.